Method and system of a wireless communication network for detecting neighbouring ues, of a first ue

ABSTRACT

A method is performed by a system of a wireless communication network, for detecting neighboring UEs of a first UE that is wirelessly connected to a first base station. The method comprises receiving power measurements performed on signals received at the first UE, which signals are sent from base stations, each received power measurement being associated with an ID of the base station from which the signal was sent, and receiving power measurements of signals received at a second UE that is wirelessly connected to a second base station, each power measurement being associated with an ID of the base station from which the signal was sent. The method further comprises determining a correlation between the received power measurements of the first and second UEs, and based on the determined correlation, determining whether the second UE is a neighbor to the first UE.

TECHNICAL FIELD

The present disclosure relates generally to methods, network nodes, userequipments, UEs, and computer programs in a wireless communicationsystem, for finding neighboring UEs with which a first UE may be able tocommunicate directly, so called device-to-device, D2D, communication, ina wireless communication system.

BACKGROUND

When D2D communication is enabled in a wireless communication system, itis required for each D2D-capable UE to find its neighbors, i.e. otherD2D-capable UEs in close physical proximity, with whom it cancommunicate directly. The wireless communication system may be e.g. aWireless Local Area Network, WLAN, or a cellular system such as WidebandCode Division Multiple Access, W-CDMA, or Long Term Evolution, LTE.

There are prior art methods for a D2D-capable UE to find its neighbors.In one type of method, as described in the published patent applicationsUS20130170470 and WO2012170794, a so called decentralized beaconingmechanism is described where a first D2D-capable UE sends a beaconsignal for other D2D-capable UEs to respond to. Based on the receivedpower of signals sent from the other D2D-capable UEs in response to thesent beacon signal, the first D2D-capable UE may calculate which of theother UEs that is the best for D2D communication.

A problem with such methods is the time it takes to discover theD2D-capable UEs. If this discovery process would take too long, it maybe useless, since no time is left for data transmission, and theneighboring UEs and the first UE may have moved since the discoveryprocess started. Also, good stopping criteria for stopping such adiscovery process are difficult to effectively define, especially sincethe surrounding environment is unknown and in an area around the firstUE there may be everything from only a few UEs to a large number of UEs.

Another problem with such a method is the high consumption of batterypower for the UEs. This is especially a problem if the amount ofpossible neighboring UEs is large. Further, an exchanging protocol hasto be defined between the D2D-enabled UEs to make them react on eachother's beaconing signals. This also implies that more network signalingwill take place in the network. There is also an increased risk of asecurity attack if a UE that fakes its ID sends and receives beaconingsignals to from other D2D-enabled UEs.

Consequently, there is a need for an improved mechanism for a firstD2D-enabled UE to discover neighboring D2D-enabled UEs. Especially,there is a need for an improved mechanism for a first D2D-enabled UE todiscover neighboring D2D-enabled UEs that are in other cells than thecell of the first UE, for example close to the first UE but on anotherside of a cell border.

SUMMARY

It is an object of the invention to address at least some of theproblems and issues outlined above. It is possible to achieve theseobjects and others by using methods, systems, UEs and computer programsas defined in the attached independent claims.

According to one aspect, a method is provided performed by a system of awireless communication network 100, for detecting neighboring UEs of afirst UE that is wirelessly connected to a first base station of thenetwork. The method comprises receiving power measurements performed onsignals received at the first UE, which signals are sent from basestations in the vicinity of the first UE, each received powermeasurement being associated with an ID of the base station from whichthe signal was sent, and receiving power measurements of signalsreceived at a second UE that is wirelessly connected to a second basestation of the network, different from the first base station, whichsignals are sent from base stations in the vicinity of the second UE,each power measurement being associated with an ID of the base stationfrom which the signal was sent. The method further comprises determininga correlation between the received power measurements of the first UEand the received power measurements of the second UE, and based on thedetermined correlation, determining whether the second UE is a neighborto the first UEa method is provided.

According to another aspect, a method is provided performed by a firstUE wirelessly connected to a first base station of a wirelesscommunication network, for detecting neighboring UEs of the first UE.The method comprises sending, to the first base station, powermeasurements performed on signals received from base stations in thevicinity of the first UE, each power measurement being associated withan ID of the base station from which the signal was received. The methodfurther comprises receiving, from the first base station, information ofone or more second UEs that is/are wirelessly connected to a differentbase station than the first base station, information indicating thatthe one or more second UEs has been determined to be a neighbor to thefirst UE based on correlations between the power measurements of thefirst UE and corresponding power measurements of individual of the oneor more second UE, and sending a signal to at least one of the one ormore second UE, based on the received information, for device to devicecommunication with the at least one of the one or more second UE.

According to another aspect, a system is provided operable in a wirelesscommunication network configured for detecting neighboring UEs of afirst UE that is wirelessly connected to a first base station of thenetwork. The system comprises a processor and a memory. The memorycontains instructions executable by said processor, whereby the systemis operative for receiving power measurements performed on signalsreceived at the first UE, which signals are sent from base stations inthe vicinity of the first UE, each received power measurement beingassociated with an ID of the base station from which the signal wassent, and receiving power measurements of signals received at a secondUE that is wirelessly connected to a second base station of the network,different from the first base station, which signals are sent from basestations in the vicinity of the second UE, each power measurement beingassociated with an ID of the base station from which the signal wassent. The system is further operative for determining a correlationbetween the received power measurements of the first UE and the receivedpower measurements of the second UE, and based on the determinedcorrelation, determining whether the second UE is a neighbor to thefirst UE.

According to another aspect, a first UE is provided, configured to bewirelessly connected to a first base station of a wireless communicationnetwork, the first UE being operable for detecting neighboring UEs ofthe first UE. The first UE comprises a processor and a memory, saidmemory containing instructions executable by said processor, whereby thefirst UE is operative for sending to the first base station, powermeasurements performed on signals received from base stations in thevicinity of the first UE, each power measurement being associated withan ID of the base station from which the signal was received. The firstUE is further operative for receiving, from the first base station,information of one or more second UEs that is/are wirelessly connectedto a different base station than the first base station, informationindicating that the one or more second UEs has been determined to be aneighbor to the first UE based on correlations between the powermeasurements of the first UE and corresponding power measurements ofindividual of the one or more second UE, and for sending a signal to atleast one of the one or more second UE, based on the receivedinformation, for device to device communication with the at least one ofthe one or more second UE.

According to other aspects, computer programs and carriers are alsoprovided, the details of which will be described in the claims and thedetailed description.

Further possible features and benefits of this solution will becomeapparent from the detailed description below.

BRIEF DESCRIPTION OF DRAWINGS

The solution will now be described in more detail by means of exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1 is an overview of an exemplary cellular communication networkshowing a possible communication scenario in which the present inventionmay be used.

FIG. 2 is a block diagram of an exemplary communication network in whichthe present invention may be used.

FIG. 3 is a flow chart illustrating a method in a system, according topossible embodiments.

FIG. 4 is another flow chart illustrating a method in a system,according to further possible embodiments.

FIG. 5 is a flow chart illustrating a method in a UE, according topossible embodiments.

FIG. 6 is an overview of another exemplary cellular communicationnetwork showing a possible communication scenario in which the presentinvention may be used.

FIG. 7 is a signaling diagram illustrating an example of a procedure,according to further possible embodiments.

FIGS. 8-9 are other signaling diagrams illustrating examples of aprocedure according to further possible embodiments.

FIGS. 10-11 are schematic block diagrams of a system according topossible embodiments.

FIGS. 12-13 are schematic block diagrams of a UE according to possibleembodiments.

DETAILED DESCRIPTION

Briefly described, a solution is provided to find neighboring UEs inother cells with which a first UE can communicate D2D. This is achieved,according to an embodiment, by a network node receiving measurementsfrom a candidate second UE in another cell than the first UE,measurements of signal quality of base station signals sent fromneighboring base stations to the second UE. These measurements arethereafter compared to measurements of signal quality of base stationsignals sent from neighboring base stations to the first UE. Thereafter,the measurements for the first and the measurements for the second UEthat are from the same base stations are compared to determine acorrelation between these comparable values. If the correlation is highenough, e.g. above a threshold, the first and the second UE aredetermined to be neighbors. In an embodiment, the measurements that areto be compared are from base stations that are within the both theneighboring cell list of the base station to which the first UE isconnected and the neighboring cell list of the base station to which thesecond UE is connected, a so called common cell list. Thereby, thenumber of signals that are to be measured and compared can be loweredcompared to if all BSs stations are analyzed.

Today, a UE is instructed to receive and measure power on pilot signalstransmitted by neighboring base stations, BS, defined in a neighbor celllist, NCL. The UE is also instructed to transmit the values of themeasured powers to its serving BS. Also, each UE and each BS have aunique local and/or global identification, ID. Since each BS and each UEhave a unique ID, the serving BS knows from which UE each measured valuecomes and for which neighboring BS it is reported. Therefore, themeasurement values from different UEs can be organized depending on towhich neighboring BS the value relates.

Further, the measurement values relating to the same neighboring BS fortwo UEs that are in the vicinity of each other are have proven to behighly correlated. Therefore, by analyzing the correlation ofmeasurement values from two UEs, an estimation of the distance betweentwo UEs can be determined. The closer the two UEs are to each other, thehigher the correlation. In an embodiment, the measurement values areorganized and sorted into power vectors, one vector for each reportingUE. In each power vector, the measurement values are sorted in a BSorder, so that measurement values for the same BS are placed in the sameposition for each power vector. A correlation metric is then calculatedbetween two vectors as an estimation of the distance between two UEs.The individual correlation metrics may then be compared with a neighborthreshold. Such a process can be repeated for all served UEs, building apool of neighbor UEs for each UE, in other words, a cluster of UEscentered on a first UE, for neighbors to the first UE. The IDs of theUEs in the pool of neighbor UEs may be stored in the BS. The IDs of theUEs in the pool may then be provided to the first UE, for example onrequest from the first UE. The first UE then has a limited set of UEswhich are more likely to be able for D2D communication in the cell andthe first UE can in an efficient way evaluate real channel conditionsfor establishing a D2D link with a suitable UE.

FIG. 1 shows an example of a wireless communication network 100 in whichthe present invention may be used. The exemplary communication systemcomprises a first BS 111 providing wireless communication to UEs beingin a geographical area of a first cell 101, and four further BSs, 112,113, 114 and 115, each BS providing wireless communication to UEs beingin a coverage area of a respective cell 102, 103, 104 and 105. In thefirst cell 101 there are in this exemplary scenario two UEs, a first UE121 and another UE 124. Further, there is a second UE 122 connected to asecond BS 112, and a third UE 123 connected to a third BS 113. Todetermine which UEs that are neighbors to each other, all four UEs121-124 are instructed by its serving BS to perform power measurementson signals, e.g. pilot signals, sent from its neighboring BSs. To limitthe number of signals that has to be analyzed for determining whetherthe first and the second UE are neighbors, the UEs are instructed toperform measurements on the signals that are coming from BSs that areboth in the neighboring cell list of the first BS 111 and in theneighboring cell list of the second BS 112.

FIG. 2 defines how the five base stations 111-115 in FIG. 1 areconnected higher up in the communication network. In this example, thecommunication network 100 is a Long Term Evolution, LTE, network,wherein all five BSs 111-115 are connected to one Mobility ManagementEntity, MME 140. However, they may be connected to different MMEs. Also,the invention is applicable in other communication networks than LTE,e.g. Global System for Mobile communication, GSM, Third Generation, 3G,Wideband Code Division Multiple Access, W-CDMA etc., in which otherentities are used in the position of the MME in the LTE network.

FIG. 3, in connection with the example of FIG. 1 illustrates a methodperformed by a system of a wireless communication network 100, fordetecting neighboring UEs of a first UE 121 that is wirelessly connectedto a first base station 111 of the network. The method comprisesreceiving 202 power measurements performed on signals received at thefirst UE, which signals are sent from base stations in the vicinity ofthe first UE, each received power measurement being associated with anID of the base station from which the signal was sent, and receiving 204power measurements of signals received at a second UE that is wirelesslyconnected to a second base station of the network, different from thefirst base station, which signals are sent from base stations in thevicinity of the second UE, each power measurement being associated withan ID of the base station from which the signal was sent. The methodfurther comprises determining 206 a correlation between the receivedpower measurements of the first UE and the received power measurementsof the second UE, and based on the determined correlation, determining208 whether the second UE is a neighbor to the first UE.

That the signals are sent from base stations in the vicinity of thefirst UE and from base stations in the vicinity of the second UE,respectively, signifies that the signals are sent from base stationswhich signals can be heard by the first UE and the second UE,respectively. This respective group of base stations may or may notcomprise the base station to which the first and the second UE isconnected. The determined correlation may be a correlation valuedefining a value of the amount of correlation between the powermeasurements of the first UE and the power measurements of the secondUE. The determining 206 of a correlation between the received powermeasurements of the first UE and the received power measurements of thesecond UE may be performed by comparing the power measurements of thefirst UE with the power measurements of the second UE so that powermeasurements of the respective first and second UE that derives from thesame base station are compared. To compare the power measurements of thefirst and the second UE that derives from the same base stationsignifies to compare the power measured at the first UE on a signaloriginating from a third base station with the power measured at thesecond UE on a signal also originating from the third base station. Thesignals from the third base station may be one and the same signal. Thepower measurements of the first and the second UE that derives from thesame base station may have been measured substantially simultaneously.

The system that performs the method may be a base station of thewireless communication network, such as the first base station 111 thatreceives the power measurements directly from the first UE. The firstbase station may then receive the measurements of the second UE via thesecond base station. Alternatively, the system that performs the methodmay be any other network node of the communication system, such as anode further away from the UE, e.g. a node in the core network or a nodein the radio access network, such as another base station, a radionetwork controller, RNC, an MME etc. In this alternative, the firstand/or second base station communicates the power measurements to thenetwork node performing the method. Alternatively, the system thatperforms the method may be a group of network nodes, whereinfunctionality for performing the method are spread out over differentphysical, or virtual, nodes of the network. The latter may be called a“cloud-solution”.

The received power measurements of the first UE may be associated withan ID of the first UE and the received power measurements of the secondUE may be associated with an ID of the second UE so that the powermeasurements are associated with the correct UE in the system. The powermeasurements may be signal strength measurements. The power measurementsmay be Reference Signal Received Power, RSRP, and/or Reference SignalReceived Quality, RSRQ. Base stations in the vicinity of thefirst/second UE may also be called neighboring base stations to thefirst/second UE. The information that is triggered to be sent to thefirst UE may be an ID of the second UE.

By such a method it can be determined whether a second UE is a neighborto a first UE. When such a determination is performed for many, e.g.all, UEs in the vicinity of the first UE, it is possible to limit thenumber of UEs that a first UE should try to communicate device to devicewith to the number of UEs including the first UE for which it hasalready been determined by the network that they are a neighbor to thefirst UE. By such a measure it is possible for the first UE to save itsbattery capacity. Also, the amount of communication from the first UE topossible D2D communication possible other UEs is limited so that networkresources are saved.

In FIG. 4 different embodiments of the method of FIG. 3 are described,the dashed boxes signifies an optional embodiment. The steps may not beperformed in the exact same order as described in the flow chart.According to an embodiment, the method described in FIG. 3 furthercomprises determining 203 a common group of base stations comprising theones of the base stations in the vicinity of the first UE and the onesof the base stations in the vicinity of the second UE that are in thevicinity of both the first UE and the second UE, and wherein thedetermining of correlations is performed only for the common group ofbase stations. The determining 203 may be performed e.g. after the powermeasurements have been received, box 203 a. Thereby, the calculationscan be limited to the measurements relevant only for the common group ofbase stations. Consequently, the calculation effort is limitedconsiderably compared to e.g. if all neighbors to the first and secondbase stations are taken into account. Alternatively, the determining 203may be performed before the power measurements have been received. Inthe latter case, the first and second UE may be instructed to onlyperform the power measurements on signals from base stations within thecommon group of base stations.

According to another embodiment, the common group of base stationscomprises base stations that are in both a neighboring cell list of thefirst base station and a neighboring cell list of the second basestation. The common group of base stations may be called a common celllist, CCL. In this context, a base station is controlling traffic withina cell. Consequently, there is a one-to-one relation between the basestation and the cell for which reason the terminology base station andcell is interchangeable. In the case of a physical base station sitecomprising a plurality of sector cells each run by a part of the basestation site, each part running a sector cell is to be seen as a basestation.

According to another embodiment, the receiving 202 of power measurementsperformed on signals received at the first UE and the receiving 204 ofpower measurements performed on signals received at the second UE areperformed only for the determined common group of base stations. By sucha feature it is possible to limit the number of base station signals toscan for each UE to signals from the base stations in the common groupof base stations.

According to another embodiment, the common group of base stations aredetermined 203 by establishing a neighborhood matrix comprising, in onecolumn each, a plurality of base stations that are in a neighboring celllist of the first base stations or in a neighboring cell list of thesecond base station, and, in one row each, the first and the second basestation. Further, in the first row, base stations of the plurality ofbase stations that are in the neighboring cell list of the first basestation are marked, and in the second row, base stations of theplurality of base stations that are in the neighboring cell list of thesecond base station are marked. The method further comprises determiningthe common group of base stations between the first and the second basestation by detecting the columns where there is a mark in both the firstand the second row.

“A column” could as a first alternative be a vertical line in thematrix, what is normally named a column, but in a second alternative, “acolumn” could be a horizontal line in the matrix. In the firstalternative, “a line” would be a horizontal line in the matrix and inthe second alternative, “a line” would be a vertical line in the matrix.Of course such a matrix could comprise more than two base stations, i.e.more than two rows, but the concept would be the same for any number ofrows. For example, if the matrix comprises 10 base stations, basestations that are in a neighboring cell list of any of the 10 basestations would have one column each, and the 10 base stations would haveone row each. In the same way as for the two-line example, a mark wouldbe made at each matrix position when that column base station is aneighbor to the row base station, and to determine a common group ofbase stations for a third and a fourth of the 10 different basestations, a mark should be found in both the row of the third and fourthbase station. Further, “a mark” could be a “1” in the specific matrixposition.

Such a matrix could be determined automatically from neighbor cell listinformation for a whole system. From the matrix it could then veryeasily be determined which base stations that are in a common cell list,for example the determining may be made automatically, by analyzing thedifferent matrix positions.

In the following is an example illustrating the above embodiment. Thecommon neighboring BSs of the first BS and the second BS are to befound. In a matrix, we then insert all BSs, BS3 to BS14, that are eithera neighbor to the first or the second BS, so that they are assigned onecolumn each. In a similar way, the first and the second BS are assignedone row each. In the first row, the BSs that are a neighbor to the firstBS is assigned a “1” in the matrix. Similarly, in the second row, theBSs that are a neighbor to the second BS is assigned a “1” in thematrix. We will then end up with the following matrix

$\begin{matrix}1 & 1 & 0 & 1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 \\0 & 1 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0\end{matrix}$

This matrix signifies that BS 1, which has row 1, has the followingneighbors: (columns from left to right) BS3, BS4, BS6, BS7, BS9, BS10.Further, BS 2, which has row 2, has the following neighbors: BS4, BS6,BS7, BS9, BS12, and BS13. When analyzing the matrix it is easilydetected which BSs that are neighbors to both BS1 and BS2 by detectingwhere it is a “1” in the same column in both rows. Consequently, in thisexample, BS4, BS6, BS7 and BS9 are in the common cell list.

In the following is another kind of matrix developed, in which therelation between a plurality of BSs, e.g. all BSs in a communicationnetwork is determined. According to this embodiment, the common group ofbase stations are determined by establishing a neighborhood matrixcomprising in one column each, the first base station, the second basestation and a plurality of further base stations in the vicinity of thefirst or the second UE, and, in one row each, the first base station,the second base station and the plurality of further base stations inthe vicinity of the first or the second UE. Then, for each row, the basestations that are in a neighboring cell list of that row's base stationare marked, e.g. with a “1”. Thereafter, the common group of basestations between the first and the second base station is determined bydetecting the columns where there is a mark in both the first and thesecond row. Also, from such a matrix other common groups of basestations can be determined, for example, the common group of basestations between BS3 and BS4 or a group of base stations common to morethan two BSs, e.g. BS1, BS2 and BS3.

In the following is an example illustrating the above embodiment. In amatrix, we then insert all BSs, in this example 12 BSs, named BS1 toBS12, for which we would like to determine common cell lists. Forsimplicity, we may say that these 12 BSs are all BSs in a network. Thefirst row and the first column is assigned to BS1, the second row andthe second column is assigned to BS2, etc. In the first row, the BSsthat are a neighbor to the BS1 is assigned a “1” in the matrix.Similarly, in the second row, the BSs that are a neighbor to BS2 isassigned a “1” in the matrix, etc. for all 10 BSs. Since one and thesame BS cannot be a neighbor to itself, a diagonal from position 1,1 toposition 12,12 will be assigned a “0”. We will then end up with thefollowing matrix:

$\begin{matrix}0 & 1 & 0 & 1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 0 \\1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 \\0 & 0 & 0 & 0 & 1 & 1 & 0 & 1 & 1 & 1 & 0 & 1 \\1 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 0 & 1 \\1 & 1 & 1 & 0 & 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 \\0 & 0 & 1 & 0 & 0 & 0 & 1 & 1 & 1 & 0 & 1 & 1 \\1 & 1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 & 1 & 1 & 1 \\1 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 0 \\0 & 0 & 1 & 1 & 1 & 1 & 0 & 1 & 0 & 1 & 0 & 1 \\0 & 1 & 1 & 1 & 0 & 0 & 1 & 1 & 1 & 0 & 1 & 1 \\0 & 1 & 0 & 0 & 1 & 1 & 1 & 0 & 0 & 1 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 1 & 1 & 0 & 1 & 1 & 0 & 0\end{matrix}$

This matrix signifies that BS 1, which has row 1, has the followingneighbors: (columns from left to right) BS2, BS4, BS5, BS7, BS8.Further, BS 2, which has row 2, has the following neighbors: BS1, BS4,BS5, BS7, BS10, BS11. When analyzing the matrix it is easily detectedwhich BSs that are neighbors to the same two BSs by detecting where itis a “1” in the same column in rows for both BSs. Consequently, in thisexample, BS4, BS5 and BS7 are in the common cell list for BS1 and BS2.As another example, BS4 and BS7 are in the common cell list for BS1 andBS12.

According to another embodiment, the method further comprises sorting205 the received measurements into power vectors, a first power vectorfor the first UE comprising the power measurements of the first UE in abase station order, and a second power vector for the second UEcomprising the power measurements of the second UE in the base stationorder. Further, the determining 206 of correlation comprises comparingthe measurements of the power vector of the first UE with themeasurements of the power vector of the second UE.

According to another embodiment, the correlation is determined 206 by anormalized scalar product between the power vector of the first UE andthe power vector of the second UE.

According to another embodiment, the method further comprises comparing207 the determined correlation to a correlation threshold. When thecorrelation is above the correlation threshold, according to thecomparison, it is determined 208 that the second UE is a neighbor to thefirst UE.

According to another embodiment, the method further comprises, when itis determined 208 that the second UE is a neighbor to the first UE,triggering sending 210 of information to the first UE that the second UEis a neighbor to the first UE.

According to another embodiment, the method further comprises, when itis determined that the second UE is a neighbor to the first UE,triggering sending 211 the determined correlation of the second UE tothe first UE.

FIG. 5, in connection with FIG. 1, shows an embodiment of a methodperformed by a first UE 121 wirelessly connected to a first base station111 of a wireless communication network 100, for detecting neighboringUEs of the first UE. The method comprises sending 302 to the first basestation 111 power measurements performed on signals received from basestations 112, 113, 114, 115 in the vicinity of the first UE, each powermeasurement being associated with an ID of the base station from whichthe signal was received. The method further comprises receiving 304,from the first base station 111, information of one or more second UEs122, 123 that is/are wirelessly connected to a different base station112, 113, 114 than the first base station 111, information indicatingthat the one or more second UEs has been determined to be a neighbor tothe first UE based on correlations between the power measurements of thefirst UE 121 and corresponding power measurements of individual of theone or more second UE 122, and sending 306 a signal to at least one ofthe one or more second UE, based on the received information, for deviceto device communication with the at least one of the one or more secondUE. Such a method makes it possible for the first UE to know about otherUEs in its vicinity with which it is plausible that it could talk D2D,before it has sent any signals itself trying to find any such UEs.According to an embodiment, the information of the one or more secondUEs comprises identities of the one or more second UEs.

According to another embodiment, the information of the one or moresecond UEs indicates a priority order in which the first UE is tocontact the one or more second UEs. According to another embodiment, theinformation indicating a priority order is the correlations between thepower measurements of the first UE and the corresponding powermeasurements of the individual of one or more second UE.

In the following, embodiments of the invention are further described.Consider a certain region with B cells. Also, assume that each cell is asite and has a Base Station, BS, which signifies that there exists B BSsover such a region. This corresponds to a multi-cell scenario where allBSs are assumed to be connected through dedicated links for datacontrol, as the case of X2 interface in LTE. Further, each BS knows itsneighbor BSs, which are stored in a list, commonly described as NeighborCell List, NCL, or a monitored set. The existence of those lists, orsimilar lists, is essential in any cellular network to ensure servicecontinuity during handover, or for cell-reselection procedures. Hence,all UEs in the cellular network are instructed to perform powermeasurements on the pilots of its serving BS and BSs in its vicinity,i.e., BSs within the NCL, and report back those values to its servingBS.

As an example, suppose that a UE served by a BS needs to perform abroadcast/multicast service request, (see 1.1 in FIG. 7) e.g., sends amessage that needs to reach all UEs in the neighborhood. Also, assumethat all UEs are D2D-capable and D2D communication mode is preferableover the traditional cellular mode. The serving BS will then constructand provide the list of candidate neighboring BSs to such UE so that itcan more effectively evaluate the real channel conditions, e.g., througha simple beacon/acknowledge mechanism, ensuring that the list providedby the serving BS is valid and direct communication is possible.

In the following, the neighborhood matrix construction will be detailed.Step 1: Building Common Cell Lists. It is possible to build a binarymatrix B that establishes the neighborhood relations between all BSs inthe scenario, so that if b_(i,j) is a logical 1, BSs i and j areneighbors and a logical 0 if i and j are not considered neighbors. Thus,one has

${B = \begin{pmatrix}b_{1,1} & b_{1,2} & \cdots & b_{1,B} \\b_{2,1} & b_{2,2} & \cdots & b_{2,B} \\\vdots & \vdots & \ddots & \vdots \\b_{B,1} & b_{B,2} & \cdots & b_{B,B}\end{pmatrix}},{with}$ $b_{i,j} = \left\{ \begin{matrix}{1,} & {{{{if}\mspace{14mu} {BS}_{i}} \in {{{NCL}_{j}\mspace{14mu} {and}\mspace{14mu} i} \neq j}},} \\{0,} & {{{otherwise}.}\mspace{166mu}}\end{matrix} \right.$

In the matrix, BS_(i) is the i-th BS (i-th column of B) and NCL_(j)(j-th row of B) is the set of BSs that are neighbors of BS_(j), and i,j∈{1,2, . . . , B}. Additionally, for i≠j, BS_(i)∈NCL_(j) implies thatBS_(j)∈NCL_(i), or in other words, b_(i,j)=b_(j,i).

FIG. 6 shows a scenario comprising four BSs: BS1 401, BS2 402, BS3 403and BS4 404, and five UEs: UE1 411, UE2 412 and UE3 413 situated in afirst cell for connection to the BS1 401; UE4 414 situated in a secondcell for connection to BS2 402 and UE5 415 situated in a third cell forconnection to BS3 403. In this scenario, a matrix B can be representedas follows

${B = \begin{pmatrix}0 & 1 & 1 & 0 \\1 & 0 & 0 & 1 \\1 & 0 & 0 & 1 \\0 & 1 & 1 & 0\end{pmatrix}},$

This matrix signifies that BS2 402 and BS3 403 are neighbors to BS1 401,BS1 401 and BS4 404 are neighbors to BS2 402, BS1 401 and BS4 404 areneighbors to BS3 403 and BS2 402 and BS3 403 are neighbors to BS4 404.The UEs 412 and 413 are instructed to perform and report measurements onthe neighbors of BS1, i.e. BS2 and BS3, for determining whether theseUES 412 and 412 are neighbors to the UE1 411 that is within the samecell as them.

For the UE4 414 and the UE5 415 that are in a different cell than UE1411 the following process applies for determining the Common Cell List,CCL, also called intersected neighbor cell list: The CCL is defined asthe set of BSs that commonly belong to (two) different NCLs in a largegeographical area. This is easy to determine using matrix B, becauseeach row represents a different NCL. This means e.g. that BS2 402 andBS3 403 has BS1 and BS4 in their CCL. Thus, consider NCL_(j) andNCL_(j), (j-th and j′-th rows of B) from BSs j and j′, respectively. TheCCL between them is given as

${CCL}_{j,j^{\prime}} = \left\{ \begin{matrix}{{{NCL}_{j}\bigcap{NCL}_{j^{\prime}}},{{{if}\mspace{14mu} {BS}_{j^{\prime}}} \in {NCL}_{j}},} \\{{\varnothing,{{otherwise}.}}\mspace{211mu}}\end{matrix} \right.$

Or using other words, CCL_(j,j′) corresponds to the common knownelements, i.e. neighbor BSs, between BSs j and j′, and j′∈{1, 2, . . . ,B}. In this context, B_(j) is the universe of CCLs with respect toBS_(j), which for BS_(j′)∈NCL_(j), is defined as the union of allpossible CCLs, i.e.,

$\begin{matrix}{B_{j} =} & {{{CCL}_{j,1}\bigcup{CCL}_{j,2}\bigcup\cdots\bigcup{CCL}_{j,{j - 1}}\bigcup{CCL}_{j,j}\bigcup{CCL}_{j,{j + 1}}\bigcup\cdots}} \\ & {{\bigcup{CCL}_{j,B}}} \\{=} & {{\bigcup\limits_{j^{\prime} = 1}^{B}\left( {{NCL}_{j}\bigcap{NCL}_{j^{\prime}}} \right)}}\end{matrix}$

In order to make the previous concepts more clear, consider, as anotherexample, the next example of neighborhood matrix B:

${B = \begin{pmatrix}0 & 1 & 1 & 1 & 1 & 1 & 1 & 0 & 0 & 0 \\1 & 0 & 1 & 0 & 0 & 0 & 1 & 1 & 1 & 1 \\\vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\0 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 \\0 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 1 & 0\end{pmatrix}},$

From B it is seen that the neighboring cell lists for NCLx, where X isbase station nr x is: NCL₁={2,3,4,5,6,7}. NCL₂={3,7,8,9,10}; . . . ;NCL₉={2,8,10}; and NCL₁₀={2,3,9}. Also, assuming a service request hasoccurred from a UE served by BS₁, the universe of CCLs B₁ is as followsB₁=CCL_(1,1) ∪CCL_(1,2) ∪ . . . ∪CCL_(1,9)∪CCL_(1,10)={2,3,4,5,6,7}∪{3,7}∪ . . . ∪Ø∪Ø. In other words, CCL_(1,2),the CCL for BS1 and BS2 comprises BS3 and BS7. This is seen directlyfrom B when looking at the first two rows.

Further Details on Step 1. Each CCL has the following properties:CCL_(j,j′)=CCL_(j′,j); The CCL of BS_(j) with itself is the NCL_(j),i.e., if j′=j, CCL_(j,j′)=NCL_(j); If the CCL_(j,j′) is an empty set,i.e., CCL_(j,j′)=Ø, then no common pilots exist to be scanned, thereforeno neighbor UEs exist between BSs j and j′. Also, in amulticast/broadcast scenario the CCL concept can be easily extended tomore than two BSs for the detection of multi-cell neighbor UEs.

Step 2: Collecting Power Measurements and Power Vectors. After the CCLhas been determined the following procedure may apply: The BS_(j) wherethe service request had its origin then asks the other vicinity BSs toinstruct their UEs to scan the received power from the BSs within theCCL_(j,j′).

Let us denote UE_(u,j) as the UE u in the coverage area of BS j and, forconvenience, each BS serves U UEs, i.e., u∈{1, 2, . . . , U}. Therefore,the maximum number of “possible” neighbors is BU−1, i.e. all UEs in thescenario minus itself. Further, the received power of each UE withrespect to each BS in the scenario may be arranged as a power matrix P,where each row represents a power vector p_(u,j) of UE_(u,j).

${P = {\begin{pmatrix}p_{1,1} \\p_{2,1} \\\vdots \\p_{U,1} \\p_{1,2} \\p_{2,2} \\\vdots \\p_{U,2} \\\vdots \\p_{1,B} \\p_{2,B} \\\vdots \\p_{U,B}\end{pmatrix} = {\begin{pmatrix}p_{{({1,1})},1} & p_{{({1,1})},2} & \cdots & p_{{({1,1})},B} \\p_{{({2,1})},1} & p_{{({1,1})},2} & \cdots & p_{{({2,1})},B} \\\vdots & \vdots & \ddots & \vdots \\p_{{({U,1})},1} & p_{{({U,1})},2} & \cdots & p_{{({U,1})},B} \\p_{{({1,2})},1} & p_{{({1,2})},2} & \cdots & p_{{({1,2})},B} \\p_{{({2,2})},1} & p_{{({2,2})},2} & \cdots & p_{{({2,2})},B} \\\vdots & \vdots & \ddots & \vdots \\p_{{({U,2})},1} & p_{{({U,2})},2} & \cdots & p_{{({U,2})},B} \\\vdots & \vdots & \ddots & \vdots \\p_{{({1,B})},1} & p_{{({1,B})},2} & \cdots & p_{{({1,B})},B} \\p_{{({2,B})},1} & p_{{({2,B})},2} & \cdots & p_{{({2,B})},B} \\\vdots & \vdots & \ddots & \vdots \\p_{{({U,B})},1} & p_{{({U,B})},2} & \cdots & p_{{({U,B})},B}\end{pmatrix}\begin{matrix}{\} P_{1}} \\{\} P_{2}} \\\vdots \\{\} P_{B}}\end{matrix}}}},{and}$ $p_{{({u,j})},b} = \left\{ \begin{matrix}{p_{{({u,j})},b},{{{if}\mspace{14mu} {UE}_{u,j}\mspace{14mu} {exists}\mspace{14mu} {and}\mspace{14mu} {BS}_{b}} \in {CCL}_{j,j^{\prime}}},} \\{{0,{otherwise},}\mspace{320mu}}\end{matrix} \right.$

where p_((u,j),b) represents the received power of UE_(u,j) from BS_(b),b∈{1, 2, . . . , B} or, using other words, for UEs served by BS_(j) orBS_(j′), the received power from BS_(b) is only considered if BS_(b)belongs to CCL_(j,j′). Moreover, P₁, P₂, . . . , P_(B) represent thepower sub-matrices in the coverage area of BS₁, BS₂, . . . , BS_(B),i.e., the single-cell case.

Further Details on Step 2. As mentioned, a kind of power measurementprocedure is mandatory in any cellular network because of the mobilityrequest and cell-reselection, due to, e.g., handover reasons. Noticethat the method described in this disclosure is easily extendable toother systems, such as Wireless Local Area Network, WLAN, and WorldwideInteroperability for Microwave Access, WiMAX, since most systems alreadydispose of methods to measure the received power, which is used e.g.during connection establishment.

In the case of LTE standards, the power measurement may be the RSRP orthe RSRQ. Notice that whenever the received power for a specific BScannot be measured by a certain UE, due to any particular reason, astandard value, e.g. zero, can be used to fill the corresponding gap inthe power vector so that the matrices are still applicable. Also, bothUE and cell IDs are assumed to be unique, and for LTE or LTE-Advancednetworks, the ID of each UE can be obtained with a DemodulationReference Signal, DMRS, which is sent in the uplink direction, and thecell ID with the Physical-layer Cell ID, PCI, transmitted in downlink.In order not to bias/polarize the results, the received power from theserving BS may be removed, i.e., set to zero, from the power vector ofeach UE because the order of magnitude of the received power isnoticeable higher than the one received from vicinity BSs, with theprevious formulation: if b=j′=j, then p_((u,j),b)=0.

The power matrix P, as presented before, can be seen as a distributeddatabase, in each BS, and can be used as required. For example, oldervalues may be ignored or replaced by new measures and the exchanging ofparts of P between BSs can be done in accordance with the changes sincelast update, therefore reducing signaling. Note that, in principle, thepower matrix P is composed by sparse vectors, e.g. with lot of zeros,depending on the number of BSs to be scanned, which may be used toimprove the correlation metric calculation, presented in the nextsection. In practice, the U UEs that are assumed in each cell is themaximum number of UEs that a BS may serve in the presented scenario,which is limited by the BS capacity.

Step 3: Building the Neighborhood Matrix. When values have beenorganized in the form of power vectors, a correlation metric is used todetermine their correlation. Therefore, when taking two different powervectors and defining a correlation threshold P_(TH), two cases mayhappen: Either the correlation is high, i.e., the correlation metric isabove the threshold P_(TH). In this situation, UEs are consideredneighbors because their set of measurements is very similar and,therefore, it is likely to happen that they are in physical proximity;or the correlation is low, i.e., the correlation metric is below thethreshold P_(TH). In this situation, UEs are not considered as neighborsbecause their set of measurements is not similar and, therefore, it islikely to happen that they are far away from each other.

Thus, the next step is to calculate the, possibly normalized,cross-correlation metric between all power vectors in the scenario. Apossible metric is defined as

${\rho_{x,x^{\prime}} = {\langle\left. \frac{x}{\left. ||x \right.||} \middle| \frac{x^{\prime}}{\left. ||x^{\prime} \right.||} \right.\rangle}},$

in which ρ_(x,x′) is the normalized cross correlation value for powervectors x=p_(x) and x′=p_(x′), with x=(u,j) and x′=(u′,j′), i.e., thecontents of two different rows of matrix P. Also,

·|·

is the inner product and ∥·∥ is the l₂ norm, defined as

${{\langle\left. x \middle| x^{\prime} \right.\rangle} = {{\sum\limits_{k = 1}^{K}\; {x_{k}x_{k}^{\prime}}} = {{x_{1}x_{1}^{\prime}} + {x_{2}x_{2}^{\prime}} + \cdots + {x_{K}x_{K}^{\prime}}}}},{\left. {and}||v \right.|| = \sqrt{\sum\limits_{k = 1}^{K}\; \left| v_{k} \right|^{2}}},{{{with}\mspace{14mu} v} = \left\lbrack {v_{1}\mspace{14mu} v_{2}\mspace{14mu} \ldots \mspace{14mu} v_{K}} \right\rbrack^{T}},{v \in \left\{ {x,x^{\prime}} \right\}},{{{and}\mspace{14mu} K} = {B.}}$

Moreover, since p_(u,j) and p_(u′,j′) are composed by non-negativequantities, ρ_((u,j),(u′,j′)) will range between 0, which meansnon-correlated, and 1, which means very highly correlated. As such, athreshold P_(TH), between 0 and 1, may be imposed, against whichdifferent values ρ_((u,j),(u′,j′)) are compared.

Finally, a neighborhood matrix may be constructed and stored in each BSas follows: If ρ_((u,j),(u′,j′)) is above the threshold(ρ_((u,j),(u′,j′))>P_(TH)), UE_(u,j) and UE_(u′,j′), i.e., UE u of BS jand UE u′ of BS j′, are tagged as neighbors, and ρ_((u,j),(u′,j′)) isplaced in the corresponding (u,j),(u′,j′) and (u′,j′),(u,j) indexes(note that ρ_((u,j),(u′,j′))=ρ_((u′,j′),(u,j)). Else, UE_(u,j) andUE_(u′,j′) are tagged as non-neighbors, and a 0 is placed in thecorresponding (u,j),(u′,j′) and (u′,j′),(u,j) indexes.

Therefore, the corresponding UE neighborhood matrix Ω looks like

${\Omega = \begin{pmatrix}w_{{({1,1})},{({1,1})}} & w_{{({1,1})},{({2,1})}} & \cdots & w_{{({1,1})},{({U,1})}} & \cdots & w_{{({1,1})},{({1,B})}} & w_{{({1,1})},{({2,B})}} & \cdots & w_{{({1,1})},{({U,B})}} \\w_{{({2,1})},{({1,1})}} & w_{{({2,1})},{({2,1})}} & \cdots & w_{{({2,1})},{({U,1})}} & \cdots & w_{{({2,1})},{({1,B})}} & w_{{({2,1})},{({2,B})}} & \cdots & w_{{({2,1})},{({U,B})}} \\\vdots & \vdots & \ddots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots \\w_{{({U,1})},{({1,1})}} & w_{{({U,1})},{({2,1})}} & \cdots & w_{{({U,1})},{({U,1})}} & \cdots & w_{{({U,1})},{({1,B})}} & w_{{({U,1})},{({2,B})}} & \cdots & w_{{({U,1})},{({U,B})}} \\\vdots & \vdots & \ddots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots \\w_{{({1,B})},{({1,1})}} & w_{{({1,B})},{({2,1})}} & \cdots & w_{{({1,B})},{({U,1})}} & \cdots & w_{{({1,B})},{({1,B})}} & w_{{({1,B})},{({2,B})}} & \cdots & w_{{({1,B})},{({U,B})}} \\w_{{({2,B})},{({1,1})}} & w_{{({2,B})},{({2,1})}} & \cdots & w_{{({2,B})},{({U,1})}} & \cdots & w_{{({2,B})},{({1,B})}} & w_{{({2,B})},{({2,B})}} & \cdots & w_{{({2,B})},{({U,B})}} \\\vdots & \vdots & \ddots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots \\w_{{({U,B})},{({1,1})}} & w_{{({U,B})},{({2,1})}} & \cdots & w_{{({U,B})},{({U,1})}} & \cdots & w_{{({U,B})},{({1,B})}} & w_{{({U,B})},{({2,B})}} & \cdots & w_{{({U,B})},{({U,B})}}\end{pmatrix}},{with}$$w_{{({u,j})},{({u^{\prime},j^{\prime}})}} = \left\{ \begin{matrix}{\rho_{{({u,j})},{({u^{\prime},j^{\prime}})}},{{{if}\mspace{14mu} \left( {u,j} \right)} \neq {\left( {u^{\prime},j^{\prime}} \right)\mspace{14mu} {and}\mspace{14mu} \rho_{{({u,j})}{({u^{\prime},j^{\prime}})}}} > P_{TH}},} \\{{0,{{otherwise}.}}\mspace{419mu}}\end{matrix} \right.$

Note that, the square matrix Ω is symmetric, i.e., values in(u,j),(u′,j′) and (u′,j′),(u,j) are equal and, in principle, sparse,i.e. with lots of zeros, thus only values different from zero in thelower or upper triangles may be used, e.g. for saving storage space.These properties are clearly shown next, where n can be rewritten as

${\Omega = \begin{pmatrix}\Omega_{1,1} & \Omega_{1,2} & \cdots & \Omega_{1,B} \\\Omega_{2,1} & \Omega_{2,2} & \cdots & \Omega_{2,B} \\\vdots & \vdots & \ddots & \vdots \\\Omega_{B,1} & \Omega_{B,2} & \cdots & \Omega_{B,B}\end{pmatrix}},$

where, e.g., Ω_(1,1) and Ω_(1,B)=Ω_(B,1) ^(T) look like

${\Omega_{1,1} = \begin{pmatrix}w_{{({1,1})},{({1,1})}} & w_{{({1,1})},{({2,1})}} & \cdots & w_{{({1,1})},{({U,1})}} \\w_{{({2,1})},{({1,1})}} & w_{{({2,1})},{({2,1})}} & \cdots & w_{{({2,1})},{({U,1})}} \\\vdots & \vdots & \ddots & \vdots \\w_{{({U,1})},{({1,1})}} & w_{{({U,1})},{({2,1})}} & \cdots & w_{{({U,1})},{({U,1})}}\end{pmatrix}};$ ${\Omega_{1,B} = \begin{pmatrix}w_{{({1,1})},{({1,B})}} & w_{{({1,1})},{({2,B})}} & \cdots & w_{{({1,1})},{({U,B})}} \\w_{{({2,1})},{({1,B})}} & w_{{({2,1})},{({2,B})}} & \cdots & w_{{({2,1})},{({U,B})}} \\\vdots & \vdots & \ddots & \vdots \\w_{{({U,1})},{({1,B})}} & w_{{({U,1})},{({2,B})}} & \cdots & w_{{({U,1})},{({U,B})}}\end{pmatrix}};$ $\Omega_{B,1} = {\begin{pmatrix}w_{{({1,B})},{({1,1})}} & w_{{({1,B})},{({2,1})}} & \cdots & w_{{({1,B})},{({U,1})}} \\w_{{({2,B})},{({1,1})}} & w_{{({2,B})},{({2,1})}} & \cdots & w_{{({2,B})},{({U,1})}} \\\vdots & \vdots & \ddots & \vdots \\w_{{({U,B})},{({1,1})}} & w_{{({U,B})},{({2,1})}} & \cdots & w_{{({U,B})},{({U,1})}}\end{pmatrix}.}$

Generally speaking, sub-matrix Ω_(j,j′)=Ω_(j′,j) ^(T) translates theneighborhood relation between UEs served by BS_(j) and BS_(j′).Additionally, j′=j represents the single-cell case.

Further Details of Step 3. The proposed normalized cross correlationmetric expressed as the normalized inner product between two powervectors is one possible correlation metric, but other correlationmetrics may be used, that can be easily found in literature. An exampleof such other correlation metric is the Pearson product-momentcorrelation coefficient or the metrics stated in section 9.6 of thebook: S. R. Saunders and A. A. Zavala, Antennas and Propagation forWireless Communication Systems, 2nd ed. John Wiley & Sons, Ltd., 2007,isbn: 978-0-470-84879-1 (e.g., Pearson's correlation coefficient).Similarly, instead of storing the real correlation value in the UEneighborhood matrix Ω, “1” may be stored wheneverρ_((u,j),(u′,j′))>P_(TH) and so obtain a binary matrix Ω. However, bystoring the real values, they can be used to sort the list of candidateneighbors of a UE, e.g. highest correlation first, and that informationmay be used to improve routing protocols in multicast or broadcastscenarios.

Additionally, instead of setting the value to zero when the powermeasure is unavailable, other reference value might be used, such as themaximum long-term fading value towards the first ring of interferingcells. Furthermore, this value can be controlled in a way that it limitsthe number of false neighbors and maximizes the number of realneighbors. Moreover, instead of using local indexes for UEs, a function,which e.g. uses BSs indexes, may be defined for translating/mappinglocal UE indexes into global ones, making the notation of n simpler.

Step 4. Store and Exchange the UE Neighborhood Matrix. Next, the builtUE neighborhood matrix is stored at the BS and the corresponding row orthe full matrix is delivered to UEs upon request.

Thereafter, to make sure that two or more D2D-capable UEs are realneighbors, i.e. to validate the information provided by the serving BSthrough the neighborhood matrix, the D2D channel should preferably beevaluated before commencing a D2D communication. Basically, it may benecessary to ensure that a D2D link can effectively be established,which can be done by a single pair of beacon/acknowledgement packets.

Further Comments on Scanning Universe and Time Basis. As said before,conventionally, UEs are instructed to perform measurements within theNCL of their serving BS. In the formulation above, this concept is takenfurther and UEs may in principle measure the received power from all BBSs within the considered scenario, or at least in B=B₁∪B₂∪ . . . ∪B_(B)(note that card{B}≤B i.e., the number of elements in B is less or equalto B). However, a full scanning in the whole universe that is timeconsuming and can completely drain UE's battery may not be requireddepending on the type of service request. In fact, it may be restrictedto B_(j) (of BS_(j)) or even just few of its CCLs.

FIG. 7 describes a message flowchart between D2D-capable UEs and BSs. At1.1, a first UE, UE_(1,1) sends a service request, for e.g. a broadcastservice, where it needs to send a message that is to reach many or allits neighboring UEs that are D2D-capable, to its serving BS, the firstBS, BS₁. This request triggers BS₁ to request 1.2 the NCLs of other BSs,BS₂, . . . , BS_(B), in the vicinity of BS₁. In response, the BS₂, . . ., BS_(B) provides 1.3 their respective NCL to the BS₁. Thereafter, BS₁computes 1.4 its neighborhood matrix B₁, and from B₁ calculates theCCL_(1,2), . . . , CCL_(1,B), i.e. the CCLs for each BS in union withBS₁. The CCLs are then provided 1.5 to the respective BS and therespective BS is requested to instruct their UEs to scan, i.e. measure,the received power from the BSs within the CCL. In 1.6, the respectiveBS, BS₂, . . . , BS_(B), as well as BS₁ requests their UEs to do thescanning. As a result, the UEs, UE_(1,1), . . . , UE_(1,B) as well asUE_(1,B), . . . , UE_(U,B) provide 1.7 the power measurements to theirserving BS, BS₁ and BS2, . . . , BS_(B) and the BSs BS₂, . . . , BS_(B)provide 1.8 their UE's power measurements further to the first basestation BS₁ BS₁ then sorts 1.9 the power measurements into power vectorsand computes 1.10 the UE neighborhood matrix Ω defining which UEs thatare neighbors. The list of neighboring UEs is then provided 1.11 to thefirst UE, UE_(1,1). The first UE then evaluates 1.12 the actualpossibility to communicate with the defined neighboring UEs, by e.g.sending a message and waiting for an acknowledgement from the respectiveUE. Here UE_(U,1) represents a UE within the same cell as UE₁, andUE_(1,B), . . . , UE_(U,B) represents UEs in other neighboring cells.

Possible execution times for the different phases may be as follows:phases 1.2 to 1.8 can be done at every 200 ms to 1 s, or 200 to 1000Transmission Time Intervals, TTIs. Phases 1.9 to 1.12 can be done ateach new service request or whenever the UE is scheduled.

FIG. 8 shows an embodiment for processing the UE neighborhood matrix Ωamong a first BS, BS_1 and a second BS, BS_2. This embodiment describesa centralized processing, which is similar to the process described inFIG. 7. In this embodiment, the common cell list, CCL {1,2} that defineswhich neighboring BSs are neighbors to both BS_1 and BS_2, has alreadybeen exchanged between BS_1 and BS_2. As already explained, CCL{1,2}=CCL {2,1}. Thereafter, BS_1 and BS_2 instructs its UEs to performmeasurements on signals from the neighboring BSs in the CCL, receivesthe power measurements from its respective UEs and creates powermatrices P for the power measurements they have respectively receives.Then the power matrix P_2 that BS_2 has created is sent to BS_1. Fromthe power matrices from BS_1, P_1, and from BS_2, P_2, BS_1 computes aUE neighborhood matrix Ω{1,2} that defines correlation values betweenUEs in cell1 and cell2. The UE neighborhood matrix Ω{1,2} (called“neighboring matrix” in FIG. 8) is then sent to BS_2. BS_1 and BS_2 thenbroadcast the UE neighborhood matrix to the UEs inside its respectivecell, at least to the D2D-enabled UEs.

FIG. 9 is similar to FIG. 8. However, here the processing of Ω isdistributed between BS_1 and BS_2 whereas in FIG. 8 the processing of Ωwas centralized to BS_1. The difference between FIG. 8 and FIG. 9 isthat BS_1 sends its power matrix P_1 to BS_2 that based on P_1 and itsown power matrix P_2 computes the UE neighboring matrix Ω{2,1}, which itbroadcasts to the UEs inside its cell. In a similar way, BS_2 sends itspower matrix P_2 to BS_1 that based on P_1 and P_2 computes the UEneighboring matrix Ω{1,2} and distributes it to the UEs inside its cell.In the latter two equal computations are performed since Ω {1,2}=Ω{2,1}.

Embodiments of the described invention is able to operate in FrequencyDivision Duplex, FDD, full duplex, and/or Time Division Duplex, TDD,modes and is not dependent on the radio access technology.

Further, given that a set of UE neighborhood matrices has been built atthe BSs, the number of UE candidate neighbors can become known to eachUE and, therefore, the time to discover a neighboring UE that the UE cancommunicate D2D with is just the time to evaluate the real channelconditions for the D2D links of the candidate neighbors. Further, thestopping criteria, i.e., the criteria that sets when the UE neighbordiscovery process shall stop, can be better defined/adjusted.

A further possible advantage is that the UE power that is consumed inthe discovery process is lower than without the above embodimentsbecause the main work for discovering neighboring UEs is done at the BS,building the neighborhood relations, which for each UE is considered tobe a small set. Thus, just before the session establishment, thecluster-head UE just needs to ensure that D2D communication can beestablished through the UE-UE channel, which can be done, e.g., by asingle pair of beacon/acknowledgement packets.

A further advantage is that no new protocol is required to beimplemented for the measurement process because signal qualitymeasurements are already available/exchanged in the network and arealready mandatory for the network to operate. Also, BSs are connectedthough a dedicated link for data control, so no new link is required.However, three new types of messages may be required: for exchanging theNCL lists between BSs; for the range of BSs to be scanned; and for eachBS to inform the neighbor candidate list to each UE.

Embodiments of the present invention solves the hidden UE problem.Building the neighborhood matrix is a centralized process and since theBS is aware of any UE-UE direct communication that is ongoing, if anycandidate neighbor is potentially seen as “hidden”, it may just beremoved from the UE neighborhood matrix, therefore avoiding the problem.

The use of network-related security features mitigates possible fakenode attacks. The network-added neighbor discovery process is less pronefor such an attack to succeed since each D2D-capable UE needs toregister itself in the network. If any misbehaving action is detected,its ID will be stored in a blacklist and prevented to operate;

Embodiments of the present invention also have the following advantages:UEs that belong to different cells, i.e. UEs served by different BSs,are able to discover each other, which enables cross-cell border D2Dcommunications; The NCL of each BS may be especially large, e.g., incrowded areas where the deployment is based on small cells. With theintroduction of CCL concept, the number of BSs to be scanned for each UEis limited, which reduces the power wasted in the scanning, thereforeimproving the UE's batteries lifetime.

FIG. 10, in connection with FIG. 1, describes an embodiment of a system600 operable in a wireless communication network 100 configured fordetecting neighboring UEs of a first UE 121 that is wirelessly connectedto a first base station 111 of the network. The system 600 comprising aprocessor 603 and a memory 604. The memory 604 contains instructionsexecutable by said processor, whereby the system 600 is operative forreceiving power measurements performed on signals received at the firstUE, which signals are sent from base stations in the vicinity of thefirst UE, each received power measurement being associated with an ID ofthe base station from which the signal was sent, and receiving powermeasurements of signals received at a second UE that is wirelesslyconnected to a second base station of the network, different from thefirst base station, which signals are sent from base stations in thevicinity of the second UE, each power measurement being associated withan ID of the base station from which the signal was sent. The system isfurther operative for determining a correlation between the receivedpower measurements of the first UE and the received power measurementsof the second UE, and based on the determined correlation, determiningwhether the second UE is a neighbor to the first UE.

The system 600 may be a base station of the wireless communicationnetwork, such as the first base station 111. Alternatively, the systemmay be any other network node of the communication system, such as anode further away from the UE, e.g. a node in the core network or a nodein the radio access network, such as another base station, a radionetwork controller, RNC, an MME etc. In this alternative, the firstand/or second base station is arranged to communicate the powermeasurements to the system 600. Alternatively, the system 600 may be agroup of network nodes, wherein the functionality of the system isspread out over different physical, or virtual, nodes of the network.The latter may be called a “cloud-solution”.

According to an embodiment, the system is further operative fordetermining a common group of base stations comprising the ones of thebase stations in the vicinity of the first UE and the ones of the basestations in the vicinity of the second UE that are in the vicinity ofboth the first UE and the second UE. The determining of correlations isthen performed only for the common group of base stations. The commongroup of base stations may comprise base stations that are in both aneighboring cell list of the first base station and a neighboring celllist of the second base station.

According to an embodiment, the system is operative for the receiving ofpower measurements performed on signals received at the first UE and forthe receiving of the power measurements performed on signals received atthe second UE only for the determined common group of base stations.

According to an embodiment, the system is operative for determining thecommon group of base stations by establishing a base stationneighborhood matrix B comprising, in one column each, a plurality ofbase stations that are in a neighboring cell list of the first basestations or in a neighboring cell list of the second base station, and,in one row each, the first and the second base station, in which in thefirst row, base stations of the plurality of base stations that are inthe neighboring cell list of the first base station are marked, and inthe second row, base stations of the plurality of base stations that arein the neighboring cell list of the second base station are marked, anddetermining the common group of base stations between the first and thesecond base station by detecting the columns where there is a mark inboth the first and the second row.

According to an embodiment, the system is operative for determining thecommon group of base stations by establishing a base stationneighborhood matrix B comprising in one column each, the first basestation, the second base station and a plurality of further basestations in the vicinity of the first or the second UE, and, in one roweach, the first base station, the second base station and the pluralityof further base stations in the vicinity of the first or the second UE,and for each row, marking the base stations that are in a neighboringcell list of that row's base station, and determining the common groupof base stations between the first and the second base station bydetecting the columns where there is a mark in both the first and thesecond row.

According to another embodiment, the system is further operative forsorting the received measurements into power vectors, a first powervector for the first UE comprising the power measurements of the firstUE in a base station order, and a second power vector for the second UEcomprising the power measurements of the second UE in the base stationorder, and wherein the system being operative for determining ofcorrelation comprises the system being operative for comparing themeasurements of the power vector of the first UE with the measurementsof the power vector of the second UE.

According to another embodiment, the system is further operative fordetermining the correlation by a normalized scalar product between thepower vector of the first UE and the power vector of the second UE.

According to another embodiment, the system is further operative forcomparing the determined correlation to a correlation threshold, andwherein the system is operative for determining that the second UE is aneighbor to the first UE when the correlation is above the correlationthreshold.

According to another embodiment, the system is further operative fortriggering sending of information to the first UE that the second UE isa neighbor to the first UE, when it has been determined that the secondUE is a neighbor to the first UE.

According to another embodiment, the system is further operative fortriggering sending the determined correlation of the second UE to thefirst UE, when it has been determined that the second UE is a neighborto the first UE.

According to other embodiments, the system 600 may further comprise acommunication unit 602, which may be considered to comprise conventionalmeans for communicating from and/or to other nodes in the network 100,such as the first and second base stations, UEs etc. The communicationunit 602 may comprise one or more communication ports for communicatingwith the other nodes in the network, or, in case the system is a basestation, transceivers for transmitting and receiving wireless signalsfrom/to UEs. The instructions executable by said processor 603 may bearranged as a computer program 605 stored in said memory 604. Theprocessor 603 and the memory 604 may be arranged in a sub-arrangement601. The sub-arrangement 601 may be a micro-processor and adequatesoftware and storage therefore, a Programmable Logic Device, PLD, orother electronic component(s)/processing circuit(s) configured toperform the actions and/or methods mentioned above.

FIG. 11 describes another embodiment of a system 600 operable in awireless communication network 100 configured for detecting neighboringUEs of a first UE 121 that is wirelessly connected to a first basestation 111 of the network. The system 600 comprises a first receivingmodule 702 for receiving power measurements performed on signalsreceived at the first UE, which signals are sent from base stations inthe vicinity of the first UE, each received power measurement beingassociated with an ID of the base station from which the signal wassent, and a second receiving module 704 for receiving power measurementsof signals received at a second UE that is wirelessly connected to asecond base station of the network, different from the first basestation, which signals are sent from base stations in the vicinity ofthe second UE, each power measurement being associated with an ID of thebase station from which the signal was sent. The system 600 furthercomprises a first determining module 706 for determining a correlationbetween the received power measurements of the first UE and the receivedpower measurements of the second UE, and a second determining module 708for determining whether the second UE is a neighbor to the first UE,based on the determined correlation. The system 600 may further comprisea communication unit 602 similar to the communication unit of FIG. 10.

FIG. 12 shows a first UE 121 configured to be wirelessly connected to afirst base station 111 of a wireless communication network 100, thefirst UE being operable for detecting neighboring UEs of the first UE.The first UE 121 comprises a processor 803 and a memory 804. The memorycontains instructions executable by said processor, whereby the first UE121 is operative for sending to the first base station 111 powermeasurements performed on signals received from base stations 112, 113,114, 115 in the vicinity of the first UE, each power measurement beingassociated with an ID of the base station from which the signal wasreceived. The first UE 121 is further operative for receiving, from thefirst base station 111 information of one or more second UEs 122, 123that is/are wirelessly connected to a different base station 112, 113,114 than the first base station 111, information indicating that the oneor more second UEs has been determined to be a neighbor to the first UEbased on correlations between the power measurements of the first UE 121and corresponding power measurements of individual of the one or moresecond UE 122, and sending a signal to at least one of the one or moresecond UE, based on the received information, for device to devicecommunication with the at least one of the one or more second UE.

According to an embodiment, the information of the one or more secondUEs indicates a priority order in which the first UE is to contact theone or more second UEs. The information indicating a priority order isthe correlations between the power measurements of the first UE and thecorresponding power measurements of the individual of one or more secondUE.

According to other embodiments, the first UE 121 may further comprise acommunication unit 802, which may be considered to comprise conventionalmeans for communicating from and/or to other nodes in the network 100,such as the first and second base stations and other UEs. Thecommunication unit 802 may comprise one or more transceivers fortransmitting and receiving wireless signals to/from the base stationsand D2D to/from other UEs. The instructions executable by said processor803 may be arranged as a computer program 805 stored in said memory 804.The processor 803 and the memory 804 may be arranged in asub-arrangement 801. The sub-arrangement 801 may be a micro-processorand adequate software and storage therefore, a Programmable LogicDevice, PLD, or other electronic component(s)/processing circuit(s)configured to perform the actions and/or methods mentioned above. Thefirst UE may further comprise a power supply such as a battery 807 forsupplying the first UE with electrical power.

FIG. 13, in connection with FIG. 1, shows an embodiment of a first UE121 configured to be wirelessly connected to a first base station 111 ofa wireless communication network 100, the first UE being operable fordetecting neighboring UEs of the first UE. The first UE 121 comprises afirst sending module 902 for sending to the first base station 111 powermeasurements performed on signals received from base stations 112, 113,114, 115 in the vicinity of the first UE, each power measurement beingassociated with an ID of the base station from which the signal wasreceived. The first UE further comprises a receiving module 904 forreceiving, from the first base station 111 information of one or moresecond UEs 122, 123 that is/are wirelessly connected to a different basestation 112, 113, 114 than the first base station 111, informationindicating that the one or more second UEs has been determined to be aneighbor to the first UE based on correlations between the powermeasurements of the first UE 121 and corresponding power measurements ofindividual of the one or more second UE 122, and a second sending module906 for sending a signal to at least one of the one or more second UE,based on the received information, for device to device communicationwith the at least one of the one or more second UE. The first UE 121 mayfurther comprise a communication unit 802 and a battery 807 similar tothe communication unit of FIG. 12.

The computer programs 605 and 805 may respectively comprise computerreadable code means, which when run in the system/the first UE causesthe system/the first UE to perform the steps described in any of thedescribed embodiments of the respective system/first UE. The computerprogram 605; 805 may be carried by a computer program productconnectable to the processor 603; 803. The computer program product maybe the memory 604; 804. The memory 604; 804 may be realized as forexample a RAM (Random-access memory), ROM (Read-Only Memory) or anEEPROM (Electrical Erasable Programmable ROM). Further, the computerprogram may be carried by a separate computer-readable medium, such as aCD, DVD or flash memory, from which the program could be downloaded intothe memory 604; 804. Alternatively, the computer program may be storedon a server or any other entity connected to the communication networkto which the system/first UE has access via the communication unit 602;802 of the respective system and first UE. The computer program may thenbe downloaded from the server into the memory 604; 804.

Although the description above contains a plurality of specificities,these should not be construed as limiting the scope of the conceptdescribed herein but as merely providing illustrations of someexemplifying embodiments of the described concept. It will beappreciated that the scope of the presently described concept fullyencompasses other embodiments which may become obvious to those skilledin the art, and that the scope of the presently described concept isaccordingly not to be limited. Reference to an element in the singularis not intended to mean “one and only one” unless explicitly so stated,but rather “one or more.” All structural and functional equivalents tothe elements of the above-described embodiments that are known to thoseof ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed hereby. Moreover, it is notnecessary for an apparatus or method to address each and every problemsought to be solved by the presently described concept, for it to beencompassed hereby.

1. A method performed by a system of a wireless communication network,for detecting neighboring User Equipments, UEs, of a first UE that iswirelessly connected to a first base station of the network, the methodcomprising: receiving power measurements performed on signals receivedat the first UE, which signals are sent from base stations in thevicinity of the first UE, each received power measurement beingassociated with an ID of the base station from which the signal wassent, receiving power measurements of signals received at a second UEthat is wirelessly connected to a second base station of the network,different from the first base station, which signals are sent from basestations in the vicinity of the second UE, each power measurement beingassociated with an ID of the base station from which the signal wassent, determining a correlation between the received power measurementsof the first UE and the received power measurements of the second UE,based on the determined correlation, determining whether the second UEis a neighbor to the first UE.
 2. Method according to claim 1, furthercomprising: determining a common group of base stations comprising theones of the base stations in the vicinity of the first UE and the onesof the base stations in the vicinity of the second UE that are in thevicinity of both the first UE and the second UE, and wherein thedetermining of correlations is performed only for the common group ofbase stations.
 3. Method according to claim 2, wherein the common groupof base stations comprises base stations that are in both a neighboringcell list of the first base station and a neighboring cell list of thesecond base station.
 4. Method according to claim 2, wherein thereceiving of power measurements performed on signals received at thefirst UE and the receiving of power measurements performed on signalsreceived at the second UE are performed only for the determined commongroup of base stations.
 5. Method according to claim 2, wherein thecommon group of base stations are determined by establishing a basestation neighborhood matrix B comprising, in one column each, aplurality of base stations that are in a neighboring cell list of thefirst base stations or in a neighboring cell list of the second basestation, and, in one row each, the first and the second base station, inwhich in the first row, base stations of the plurality of base stationsthat are in the neighboring cell list of the first base station aremarked, and in the second row, base stations of the plurality of basestations that are in the neighboring cell list of the second basestation are marked, and determining the common group of base stationsbetween the first and the second base station by detecting the columnswhere there is a mark in both the first and the second row.
 6. Methodaccording to claim 2, wherein the common group of base stations aredetermined by establishing a base station neighborhood matrix Bcomprising in one column each, the first base station, the second basestation and a plurality of further base stations in the vicinity of thefirst or the second UE, and, in one row each, the first base station,the second base station and the plurality of further base stations inthe vicinity of the first or the second UE, and for each row, markingthe base stations that are in a neighboring cell list of that row's basestation, and determining the common group of base stations between thefirst and the second base station by detecting the columns where thereis a mark in both the first and the second row.
 7. Method according toclaim 1, further comprising: sorting the received measurements intopower vectors, a first power vector for the first UE comprising thepower measurements of the first UE in a base station order, and a secondpower vector for the second UE comprising the power measurements of thesecond UE in the base station order, and wherein the determining ofcorrelation comprises comparing the measurements of the power vector ofthe first UE with the measurements of the power vector of the second UE.8. Method according to claim 7, wherein the correlation is determined bya normalized scalar product between the power vector of the first UE andthe power vector of the second UE.
 9. Method according to claim 1,further comprising: comparing the determined correlation to acorrelation threshold, and wherein it is determined that the second UEis a neighbor to the first UE when the correlation is above thecorrelation threshold.
 10. Method according to claim 1, furthercomprising, when it is determined that the second UE is a neighbor tothe first UE, triggering sending of information to the first UE that thesecond UE is a neighbor to the first UE.
 11. Method according to claim1, further comprising: when it is determined that the second UE is aneighbor to the first UE, triggering sending the determined correlationof the second UE to the first UE.
 12. A method performed by a first UserEquipment, UE, wirelessly connected to a first base station of awireless communication network, for detecting neighboring UEs of thefirst UE, the method comprising: sending to the first base station powermeasurements performed on signals received from base stations in thevicinity of the first UE, each power measurement being associated withan ID of the base station from which the signal was received; receiving,from the first base station information of one or more second UEs thatis/are wirelessly connected to a different base station than the firstbase station, information indicating that the one or more second UEs hasbeen determined to be a neighbor to the first UE based on correlationsbetween the power measurements of the first UE and corresponding powermeasurements of individual of the one or more second UE, and sending asignal to at least one of the one or more second UE, based on thereceived information, for device to device communication with the atleast one of the one or more second UE.
 13. Method according to claim12, wherein the information of the one or more second UEs indicates apriority order in which the first UE is to contact the one or moresecond UEs.
 14. Method according to claim 13, wherein the informationindicating a priority order is the correlations between the powermeasurements of the first UE and the corresponding power measurements ofthe individual of one or more second UE.
 15. A system operable in awireless communication network configured for detecting neighboring UEsof a first UE that is wirelessly connected to a first base station ofthe network, the system comprising a processor and a memory said memorycontaining instructions executable by said processor, whereby the systemis operative for: receiving power measurements performed on signalsreceived at the first UE, which signals are sent from base stations inthe vicinity of the first UE, each received power measurement beingassociated with an ID of the base station from which the signal wassent, receiving power measurements of signals received at a second UEthat is wirelessly connected to a second base station of the network,different from the first base station, which signals are sent from basestations in the vicinity of the second UE, each power measurement beingassociated with an ID of the base station from which the signal wassent, determining a correlation between the received power measurementsof the first UE and the received power measurements of the second UE,and based on the determined correlation, determining whether the secondUE is a neighbor to the first UE.
 16. System according to claim 15,further being operative for determining a common group of base stationscomprising the ones of the base stations in the vicinity of the first UEand the ones of the base stations in the vicinity of the second UE thatare in the vicinity of both the first UE and the second UE, and whereinthe determining of correlations is performed only for the common groupof base stations.
 17. System according to claim 16, wherein the systemis operative for the receiving of power measurements performed onsignals received at the first UE and for the receiving of the powermeasurements performed on signals received at the second UE only for thedetermined common group of base stations.
 18. System according to claim16, wherein the system is operative for determining the common group ofbase stations by establishing a base station neighborhood matrix Bcomprising, in one column each, a plurality of base stations that are ina neighboring cell list of the first base stations or in a neighboringcell list of the second base station, and, in one row each, the firstand the second base station, in which in the first row, base stations ofthe plurality of base stations that are in the neighboring cell list ofthe first base station are marked, and in the second row, base stationsof the plurality of base stations that are in the neighboring cell listof the second base station are marked, and determining the common groupof base stations between the first and the second base station bydetecting the columns where there is a mark in both the first and thesecond row.
 19. System according to claim 16, wherein the system isoperative for determining the common group of base stations byestablishing a base station neighborhood matrix B comprising in onecolumn each, the first base station, the second base station and aplurality of further base stations in the vicinity of the first or thesecond UE, and, in one row each, the first base station, the second basestation and the plurality of further base stations in the vicinity ofthe first or the second UE, and for each row, marking the base stationsthat are in a neighboring cell list of that row's base station, anddetermining the common group of base stations between the first and thesecond base station by detecting the columns where there is a mark inboth the first and the second row.
 20. System according to claim 15,further being operative for sorting the received measurements into powervectors, a first power vector for the first UE comprising the powermeasurements of the first UE in a base station order, and a second powervector for the second UE comprising the power measurements of the secondUE in the base station order, and wherein the system being operative fordetermining of correlation comprises the system being operative forcomparing the measurements of the power vector of the first UE with themeasurements of the power vector of the second UE.
 21. System accordingto claim 20, wherein the system is operative for determining thecorrelation by a normalized scalar product between the power vector ofthe first UE and the power vector of the second UE.
 22. System accordingto claim 15, further being operative for comparing the determinedcorrelation to a correlation threshold, and wherein the system isoperative for determining that the second UE is a neighbor to the firstUE when the correlation is above the correlation threshold.
 23. Systemaccording to claim 15, further being operative for, when it has beendetermined that the second UE is a neighbor to the first UE, triggeringsending of information to the first UE that the second UE is a neighborto the first UE.
 24. System according to claim 15, further beingoperative for, when it has been determined that the second UE is aneighbor to the first UE, triggering sending the determined correlationof the second UE to the first UE.
 25. A first user equipment, UE,configured to be wirelessly connected to a first base station of awireless communication network, the first UE being operable fordetecting neighboring UEs of the first UE, the first UE comprising aprocessor and a memory, said memory containing instructions executableby said processor, whereby the first UE is operative for: sending to thefirst base station power measurements performed on signals received frombase stations in the vicinity of the first UE, each power measurementbeing associated with an ID of the base station from which the signalwas received; receiving, from the first base station information of oneor more second UEs that is/are wirelessly connected to a different basestation than the first base station, information indicating that the oneor more second UEs has been determined to be a neighbor to the first UEbased on correlations between the power measurements of the first UE andcorresponding power measurements of individual of the one or more secondUE, and sending a signal to at least one of the one or more second UE,based on the received information, for device to device communicationwith the at least one of the one or more second UE.
 26. First UEaccording to claim 25, wherein the information of the one or more secondUEs indicates a priority order in which the first UE is to contact theone or more second UEs.
 27. A computer program comprising computerreadable code means to be run in a system of a wireless communicationnetwork, configured for detecting neighboring UEs of a first UE that iswirelessly connected to a first base station of the network, whichcomputer readable code means when run in the system causes the system toperform the following steps: receiving power measurements performed onsignals received at the first UE, which signals are sent from basestations in the vicinity of the first UE, each received powermeasurement being associated with an ID of the base station from whichthe signal was sent, receiving power measurements of signals received ata second UE that is wirelessly connected to a second base station of thenetwork, different from the first base station, which signals are sentfrom base stations in the vicinity of the second UE, each powermeasurement being associated with an ID of the base station from whichthe signal was sent, determining a correlation between the receivedpower measurements of the first UE and the received power measurementsof the second UE, and based on the determined correlation, determiningwhether the second UE is a neighbor to the first UE. 28-29. (canceled)30. A computer program comprising computer readable code means to be runin a first UE configured to be wirelessly connected to a first basestation of a wireless communication network, the first UE being operablefor detecting neighboring UEs of the first UE, which computer readablecode means when run in the first UE causes the first UE to perform thefollowing steps: sending to the first base station power measurementsperformed on signals received from base stations in the vicinity of thefirst UE, each power measurement being associated with an ID of the basestation from which the signal was received; receiving, from the firstbase station information of one or more second UEs that is/arewirelessly connected to a different base station than the first basestation, information indicating that the one or more second UEs has beendetermined to be a neighbor to the first UE based on correlationsbetween the power measurements of the first UE and corresponding powermeasurements of individual of the one or more second UE, and sending asignal to at least one of the one or more second UE, based on thereceived information, for device to device communication with the atleast one of the one or more second UE. 31-32. (canceled)